We’ve swapped out a load of these variators over the last few weeks so thought it’d be interesting to pull one apart and try to explain the operation, which is incredibly elegant. For an overview on how the change in timing is controlled take a look here and for an in-depth look at how to get the timing of an engine spot on head over here.
The variator only has six parts which are shown below. The circlip holds everything together and the spring holds the internals in the resting/off position.
The end of the main body of the variator has the attachment points for the cambelt pulley, which in turn is fixed to the crank:
The inside of the body is machined with a helical gear profile. Behind the gear is a cylinder with a slightly larger diameter:
In the centre we have a shaft, the nose of which runs in the end of the body and the flange which runs at the back of the body. This shaft is free to rotate inside the body. It is held in place by the circlip and, apart from being able to rotate, it is fixed in relation to the body.
The rear of the shaft screws into the camshaft with a counterclockwise thread (to prevent it from undoing itself). The variator shaft is fixed in relation to the camshaft.
The variator shaft has splines that are longitudinal to the camshaft/crank.
Sitting between the shaft and the body there are two gears. The larger gear has a flange that runs inside the cylinder of the body. Note that both gears have helical teeth on the outside:
And splines to the inside:
It should be straightforward to understand that the inside of these gears run on the shaft and the outer runs inside the body.
Also note that the two gears are not affixed to each other however they do work together in harmony. I assume that they were designed as two pieces in order to prevent the helical teeth from becoming snagged up.
This is the position of the teeth relative to the shaft and body in the rest position. Note that the spring would be pushing the teeth to the right of the picture:
When the Motronic decides to activate the variator, oil is allowed to flow into the hole marked red and fills the cylinder of the body with oil under high pressure. This acts against the flange of the larger gear and moves both gears backwards along the shaft (to the left in the picture). The helical gears then turn the body of the variator in relation to the shaft, thus altering the timing.
This changes the timing of the exhaust camshaft by 10 degrees, which equates to 20 degrees at the crank since the cams are turning at half the engine speed.
If one is looking at the front of the engine then the crankshaft is turning clockwise and the camshafts are turning counterclockwise. When the variator is activated, the exhaust camshaft is moved clockwise in relation to the crankshaft. This means that the exhaust valves open later and the overlap with the intake valves is increased.
Other posts in this category: Engine and Drivetrain
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Aldous Voice 